1. Field of the Invention
The present invention relates to growing industrial-size SiC single crystals by sublimation and, more specifically, to such growth by the technique of Axial Gradient Transport (AGT).
2. Description of Related Art
Wafers of silicon carbide of hexagonal 4H and 6H polytypes serve as lattice-matched substrates to grow epitaxial layers of SiC and GaN, which are used for the fabrication of SiC- and GaN-based semiconductor devices utilized in power and microwave electronic applications.
Large SiC single crystals are grown conventionally by sublimation using the technique of Physical Vapor Transport (PVT). A schematic diagram of a common PVT arrangement is shown in FIG. 1. PVT growth is carried out in vertical crucible 11, which is generally made of graphite. Sublimation source material 13 is disposed at the bottom of the crucible, while a growing crystal (or boule) 15 grows on a seed crystal 14 disposed at the crucible top, for instance, attached to the interior of the crucible lid 12. Most commonly, inductive heating with a single RF coil is utilized for PVT growth. This heating arrangement is shown in FIG. 1 which includes a cylindrical RF coil 19 positioned coaxially with growth crucible 11.
PVT growth is carried out at temperatures generally between 2000° C. and 2400° C. In order to control the vapor transport rate, PVT growth is carried out under a small pressure of inert gas (e.g., helium and/or argon), generally between 1 Torr and 100 Torr.
At these temperatures and pressures, source material 13 vaporizes and fills the interior of crucible 11 with volatile molecular species, such as Si, Si2C and SiC2. During the growth of growing crystal 15 on seed crystal 14, the temperature of source material 13 is maintained higher than that of the seed crystal 14, typically by 10 to 200° C. This temperature difference forces the vapors to migrate and condense on seed crystal 14 causing the growth of growing crystal 15.
The quality of PVT-grown SiC crystals depends on growth conditions, such as the sign and value of radial temperature gradients in the upper part of crucible 11 where the growth of growing crystal 15 occurs. Strong temperature gradients in growing crystal 15, especially radial ones, cause thermo-elastic stress and the generation of defects and cracking in growing crystal 15.
It is known in the art of SiC sublimation growth that the crystal growth interface closely follows the shape of isotherms in the crystal and its vicinity. Positive radial gradients (where the temperature inside of the growth crucible increases in the radial direction from the crucible axis toward the crucible wall) produce a convex (toward source material 13) growth interface. Negative radial gradients (where the temperature decreases in the radial direction from the crucible axis toward the crucible wall) produce a concave (toward source material 13) growth interface. Zero radial gradient (where the temperature does not change in the radial direction from the crucible axis toward the crucible wall) produces a flat growth interface.
Curved growth interfaces, convex or concave, can lead to the appearance of crude macrosteps on the growth interface causing polytype instability and generation of defects. Accordingly, it is generally believed that a flat growth interface is the most conducive to the growth of high quality crystals, such as growing crystal 15.
Generally, the conventional PVT heating geometry shown in FIG. 1 creates an axisymmetric thermal field in crucible 11 with strong radial temperature gradients which are difficult to control.
Another problem of single-RF coil PVT heating shown in FIG. 1 is that it is difficult to scale up for the growth of larger-diameter crystals. With increase in the crucible diameter and the coil diameter, radial gradients become steeper, while electromagnetic coupling between the coil and crucible becomes less efficient.
A PVT sublimation growth technique called Axial Gradient Transport (AGT) is disclosed in U.S. Pat. No. 6,800,136 (hereinafter “the '136 patent”) and has as its goal to reduce undesirable radial temperature gradients. A conceptual diagram of the AGT growth geometry from the '136 patent is shown in FIG. 2.
The AGT technique utilizes two independent flat heaters, namely, a source heater and a boule heater. The heaters can be either inductive or resistive. The heaters are positioned coaxially with the crucible, with the source heater disposed below the source material and the boule heater disposed above the growing crystal.
The AGT technique includes means for reducing heat flow in the radial direction, desirably to zero. This means includes cylindrical thermal insulation and an additional heater disposed around the AGT growth cell. A properly adjusted combination of the cylindrical thermal insulation and the heater can reduce radial heat losses to zero. The AGT geometry shown in FIG. 2 allegedly leads to strictly axial heat flow with essentially zero radial gradients.
The AGT apparatus utilizing inductive heating is described in detail in the '136 patent, which is incorporated herein by reference. This inductively heated AGT arrangement is shown in FIG. 3. It employs two flat RF coils, namely, top coil 30a and bottom coil 30b. The cylindrical crucible 31 including source material 32 and a seed crystal 33, upon which a growing crystal 35 grows, is disposed between these coils, whereby the top and bottom of the crucible serve as flat RF susceptors. Arrows 34 signify vapor transport in the growth crucible in the direction from source to crystal.
A disadvantage of the AGT cell design shown in FIG. 3 is related to the character of RF coupling between the flat coils 30a and 30b and the flat top and bottom of the crucible 31. There are two main types of flat RF coils, commonly known as “snail” and “snake” coils. When coupled to a disk-like susceptor, a “snail” coil will deposit its RF energy mostly at the susceptor edges due to skin-effect, as shown in FIG. 3. This type of coupling leads to poorly controllable radial temperature gradients in the crucible. “Snake” coils offer better uniformity of energy deposition, but their overall coupling efficiency is low.
An AGT apparatus utilizing flat resistive heaters is also disclosed in the '136 patent. At source material sublimation temperatures, radiation is the main mechanism of heat transfer from the heater to the crucible. Therefore, flat resistive heaters should be free from the disadvantages of flat RF coils.
A simple resistively heated AGT arrangement is shown in FIG. 4A. The cylindrical crucible 41 is placed between two flat resistive heaters 40a and 40b, which are shaped as disks with their diameters larger than that of the crucible. The upper heater 40a is disposed above a seed crystal 43, upon which a growing crystal 45 grows, while the lower heater 40b is disposed below source material 42. Arrows 44 denote the direction of vapor transport in the crucible.
The arrangement of FIG. 4A has the disadvantage that it creates negative radial gradients (concave isotherms) in the vicinity of the growing crystal. This is illustrated in FIG. 4B which shows the results of finite element simulation of the AGT cell shown in FIG. 4A. The strongly concave isotherms 46 are clearly visible. The root cause of these concave isotherms 46 is radial heat losses.
To some degree, concave isotherms 46 can be reduced by increasing the thickness of cylindrical thermal insulation around the AGT growth cell and/or by using additional cylindrical heater(s), as described above in connection with FIG. 2. However, this will make such AGT growth systems prohibitively large, complex, and expensive.
For SiC sublimation growth, graphite is a natural choice of heater material. In order to achieve the required temperature inside the growth crucible (up to 2400° C.), the heater temperature should be by 100-200° higher. Stability and reliability of graphite heaters at such high temperatures are poorly studied.
One particular problem of all resistive heaters operating at high temperatures in an inert gas atmosphere is the phenomenon of thermionic emission. At high temperatures, electron clouds form around the heater. Driven by the electric field created by electric current passing through the heater, these electrons migrate in the gas-filled space and contribute to the total current between the heater terminals. With increase in the heater voltage, the electrons can acquire enough energy for gas ionization. The produced gas ions can cause secondary (cascade) gas ionization leading to glow discharge.
Glow discharge alters the heating geometry and leads to the erosion of the graphite crucible, the heater, and the thermal insulation. Also, with the onset of glow discharge, the electric current across the heater becomes unstable, thus creating growth instabilities leading to stress and defects in the growing crystal.
Gas ions accelerated by the electric field bombard the heater surface and can cause secondary electron emission. This chain of surface bombardment and ionization events at high temperatures is called thermionic emission (glow discharge is, in fact, the first stage of thermionic emission). With further increase in the heater temperature and voltage, and with a sufficient supply of gas ions, glow discharge evolves into arc. Such arc can cause severe damage to the heater, crucible and power supply. Therefore, in order to realize the advantages of resistive heating in AGT growth of SiC crystals, glow discharge in the growth system is desirably avoided.